A security document or security paper can be defined as that which has particular features assuring its origin and, therefore, its authenticity. Those security documents include travel and identification documents, bank notes, checks, stamps and stamped paper, labels or tickets, among many others.
A security element can be defined as that which is integrated in the security document in its pulp (such as, for example, the security thread, security fibers, watermarks, tactile effects or other elements commonly used) or on its surface (such as, for example, holograms added in different notes and credit cards, security inks, plastic films or other elements which are commonly used), randomly distributed or fixed in specific positions of the document and conferring security features to the document containing it, these features being able to greatly vary provided that their purpose is to hinder the forgery of security documents or to facilitate their authentication.
The use of different security elements has expanded in recent years as is proven by the number of patents in this field.
Some of these elements are detectable directly by humans, whereas other security elements incorporated in the documents require the use of special tools for their detection. A clear example is the luminescent substances or dyes existing in various security documents which require the use of a special light (for example ultraviolet light) for observation.
The use of luminescent substances or dyes for certifying authenticity in security documents has been known for a long time now (such as, for example, German patent number 449133 from the year 1925).
The use of luminescent substances or dyes of this type for certifying the authenticity of documents presents several drawbacks. One of the greatest drawbacks is the limited amount of optical transitions (absorptions and emissions) with properties suited for this application. As described in U.S. Pat. No. 4,451,530, a security dye or substance must show optical emissions centered in a well-defined frequency and preferably in a narrow band. These properties are present in the family of so-called rare earth elements (lanthanides with a valence 3+), as shown in the book “An introduction to the Optical Spectroscopy of Inorganic Solids” (J. García Solé, L. E. Bausá, and D. Jaque, © 2005 John Wiley & Sons, Ltd ISBNs: 0-470-86885-6 (HB); 0-470-86886-4 (PB)). The optical properties of all the rare earth elements are tabulated in Dieke's diagram [Dieke, G. H., Spectra and Energy Levels of Rare Earth Ions in Crystals, Interscience, New York (1968)], such that they are known and limited, a feature which involves a high risk of vulnerability and limits the scope of application because they cannot be used as a coded element.
The use of materials with a certain magnetic response for marking goods is also well known. One of the possible applications is the use of soft magnetic materials presenting hysteresis cycles with a large Barkhausen discontinuity, such as those described in patent U.S. Pat. No. 7,336,215 B2. In this case they are wires made of a soft magnetic material, generally based on metals or metal alloys, with diameters of several tenths of a millimeter.
Anti-theft markers based on magnetic responses are generally fairly large, having sizes of several millimeters or centimeters, and are coupled in different ways to the products to be protected. U.S. Pat. No. 4,484,184 describes one of these anti-theft markers based on the use of soft magnetic materials with a particular response to an oscillating magnetic field at a determined frequency. This marker has a size of several millimeters, making its presence detectable to the naked eye. In addition, in general these anti-theft markers usually have a performance that relies heavily upon their shape, such that when they are deformed thy may lose their functionality.
Anti-theft magnetic marking systems are based on producing electromagnetic fields with a determined frequency in response to an interrogating alternating electromagnetic field, which allows remote detection in a simple manner. These markers can be activated and deactivated by applying a suitable magnetic field for that purpose. This feature is very useful for marking products in a shop and preventing theft but limits its use as a marker in a security document.
There are different patents relating to systems for detection for security elements operating in the microwave range. In this sense, patent WO 9927502 describes a portable-sized microwave detector for particles acting as electric dipoles. The device measures the reflection and transmission of a radiofrequency wave striking the document containing the aforementioned particles, and it performs different operations with the measurements obtained, comparing with values which require being stored previously as reference values, in order to determine the authenticity of the document. However this system for detection is not robust enough and can give false positives.